1. Field of the Invention
The invention relates to a two wire current transmitter where current through a DC source and load is controlled by the transmitter to correspond with the magnitude of a value of a parameter to be sensed by a sensing element, which may typically be a thermocouple wherein DC isolation is provided, or a temperature sensitive resistor without isolation.
2. Prior Art
U.S. Pat. No. 3,573,599 issued Apr. 6, 1971, discloses a transformer coupling to provide DC isolation to a sensing circuit from a supply circuit in a two wire current transmitter. The sensing circuit includes AC amplifier, whose input repetitively samples and compares the signals of the DC sensor network from the DC feedback network which is transformer coupled to the supply circuit. A DC amplifier on the supply side acts as the current controller for the supply current. Multiple AC coupling means are required between a sensing element and a supply current.
A two wire transmitter is described in U.S. Pat. No. 3,764,880 issued Oct. 9, 1973, wherein a single transformer is provided for DC isolation between a transducer circuit and a source circuit. The source circuit is connected to an input side of a voltage regulator which provides a regulated voltage to a DC to DC converter over the current range of the transmitter which may be, for example, 4-20 milliamperes. The voltage regulator of this circuit requires only a small, substantially constant operating current to provide required operating voltage regulation to the converter circuit.